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    <title>Document</title>
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    <script>
      // 变量作用域，链
      {
        console.log(1);
        let num = 100;
        {
          console.log(2);
          // let num = 200;
          {
            // let num = 201;
            num = 1000;
            console.log(3);
            console.log(num);
          }
        }
      }
    </script>
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